def letter_big(fnc):
    def wrapper(*args, **kwargs):
        ret = fnc(*args, **kwargs)
        if isinstance(ret,str):
            print(f"@wrapper has deal with the string, and change it to :{ret.capitalize()}")
            return ret.capitalize()
        raise ValueError('String must not be null.')
    return wrapper

@letter_big
def get_string(string):
    print(f"This function get string:{string}.")
    return string

if __name__ == '__main__':
    string = 'hello string'
    get_string(string)